Diseases and nematodes have been much more of a problem in 2018 in cotton than in any other year that I can remember. This was and continues to be largely due to the frequent rains and wet weather throughout much of the season. Those conditions factor the spread of fungal and bacterial diseases.
There are seven significant disease/nematode conditions present in Georgia’s cotton fields now. While there is not much growers can do or need to do about it now, they should pay attention so as to make the best management decisions in 2019.
Here’s the list:
#1 Stemphylium leaf spot.
It is present in many fields and is identified as small-to-moderate sized lesions, often encircled by a dark, purple ring, on leaves showing signs of nutrient (potassium) deficiency.
Stemphylium only occurs in conjunction with a potassium deficiency in the plant and can lead to rapid defoliation and significant yield loss. Stemphylium leaf spot is a very important problem in the state and is likely overlooked as growers have either become too familiar with it or do not think that there is much that can be done.
It typically occurs in the same areas of a field year after year – sandier areas, sometimes infected with nematodes. Grower should take special steps to manage soil fertility (and nematodes) to reduce losses to this disease.
#2. Target spot.
Target spot has been especially widespread this season because of extended periods of wet weather. As I have often said, use of fungicides is not always profitable if the level of target spot is low because of hot and dry conditions.
However, I believe most growers who protected their cotton crop with fungicides in 2018 will see some benefit in doing do. We should have some interesting data to share with the growers during the winter meeting season.
#3. Areolate mildew.
This has been problematic again for the second year in a row across a large section of Georgia’s cotton crop. I am not really sure why that is, but I do suspect that the fungus successfully survived in crop debris from last year and was brought on again by the rainy season.
I hope that this disease does not become an every-year problem for our cotton producers. At upcoming winter meetings, we should have data on fungicide control and yields.
#4. Bacterial blight.
This became established in some fields very early in the season and I had expected it to be a major problem. Statewide, bacterial blight has been a very minor issue in 2018, demonstrating that the development and spread of a disease can be difficult to predict.
Growers are reminded to be careful in their selection of varieties for 2019 as resistant varieties are THE most important measure for managing this disease.
#5. Fungal boll rots.
These are likely to be quite severe in 2018, especially in fields with excessive, rank growth. In some situations, limited defoliation from leaf diseases could actually be a good thing by opening up the canopy and allowing airflow to reduce humidity and dry the bolls. Fungicides are not an effective management tool for control of boll rot.
#6. Fusarium wilt.
It is becoming an increasing problem in Georgia’s cotton fields. I don’t know if this is because the problem is spreading or simply because growers are paying greater attention to it.
Nonetheless, at this point Fusarium wilt can ONLY be managed in our fields by managing the parasitic nematodes associated with it, often by treating the field with a nematicide. Again, we should have some excellent data to share after this field season.
In general, nematodes – root-knot, reniform, sting and lance – are a significant problem in Georgia cotton. Growers are encouraged to take the time after harvest and before cold weather hits to take soil samples from areas of poor growth in order to determine if nematodes are indeed a problem.
Taking stock of disease and nematode issues at the end of the 2018 season should help growers to make effective management decisions for 2019.